1. Field of the Invention
This invention relates to improvements in steam heated rotary dryer drums of the type used in paper making machines wherein wet paper web is dried by passing the same through a dryer which generally comprises a plurality of rotatably mounted drums over which the web is threaded for evaporating moisture from the web, and is more particularly directed to the attainment of substantially uniform heat transfer throughout the heating surface of a steam heated dryer drum equipped with spoiler bars and stationary siphon means.
2. Prior Art
In order to attain maximum production, often referred to as tonnage, in a paper making machine, every step in production must be as nearly as practicable fine tuned for maximum efficiency. Not the least of the production steps resides in the drying of the wet paper web. For high speed drying and dryer economy, it is imperative to secure the maximum drying heat from the steam supplied to the hollow dryer drums.
Conventionally, the web to be dried is held in contact with a series of the dryer drums which are generally large cast iron cylindrical shells closed at each end and steam heated and rotated at the speed of travel of the web. The steam supplied for heating the drums condenses as the heat is transferred through the cylindrical dryer shell wall. Centrifugal action causes the condensate to form a thin layer around the entire cylindrical inside surface of the drum which produce an insulating effect and reduces the rate of heat transfer from the steam through the drum to the web being dried. The reduction in drying rate thus caused is disadvantageous not only for steam economy reasons but also for heating efficiency reasons. A particular problem arises when the condensate layer is relatively thick and the desired dryer speed is high.
In order to evacuate the condensate and at the same time maintain as thin a film of the condensate as possible, siphons are provided which extend from outside of the dryer drum through rotary seals and have suction intakes located within a fraction of an inch of the inner surface of the dryer shell. These siphons may either be rotary, that is secured to the dryer shell and rotating with it, or stationary, that is with the intake suspended above the low point in the rotation of the inner surface of the dryer shell.
For efficiency the stationary form of siphon is preferred because it does not require unusual differential suction pressure since there are no centrifugal forces to overcome. However, the stationary siphon intake cannot be accurately located close to the cylindrical dryer shell wall surface because of the difficulty in providing the required mechanical rigidity through the dryer journal and rotary seal. The necessary siphon-to-shell wall clearance therefore results in a condensate layer which tends to unduly impede the heat transfer efficiency of the dryer drum. For overcoming this deficiency by reducing the effect of the rimming insulating layer of condensate numerous and varied arrangements have been proposed such as condensate removal pipes and other devices to minimize the condensate layer thickness, but it is impossible to eliminate the layer completely, and even a very thin layer of condensate will have a detrimental insulating effect.
Substantial improvement has been attained by providing the inner cylindrical surface of the dryer drum with circumferentially spaced and generally axially extending spoiler bars, exemplified in U.S. Pat. Nos. 3,217,426 and 3,724,094. These spoiler bars permit the condensate depth to be significantly thicker without impeding the heat transfer because of the resonant action of the condensate between the bars, and more particularly by virtue of the turbulence of the condensate effected by action of the bars. The spoiler bar action is especially well described and illustrated in U.S. Pat. No. 3,724,094, and to whatever exent necessary the disclosure of that patent is incorporated herein by reference. These spoiler bars are especially well suited to dryer drums with stationary siphons.
However, a problem has been encountered in the stationary siphon and spoiler bar arrangement, because the ends of the bars adjacent to the siphon cannot be placed closely to or under the intake tip of the stationary siphon without risking potential mechanical interference. Without the spoiler bars in the narrow annular heat transfer area of the drum wall which rotates past the siphon intake, there is a condensate rimming, i.e. centrifugal accumulation, effect and the heat transfer from this narrow area tends to be poor in comparison with the rest of the drying surface of the dryer drum. This causes non-uniformity in the moisture content of the paper web being dried by leaving the longitudinal area of the web along the narrow siphon intake area of the dryer drum with an undesirable moisture content compared to the remainder of the web.
A proposal for eliminating the non-uniform heat transfer problem is disclosed in U.S. Pat. No. 4,183,149, wherein instead of spoiler bars, a brush is mounted to extend longitudinally throughout the width of the effective heat transfer surface of the drum. Such brush is mounted stationarily within the drum and circumferentially spaced from the vertical disposition of the siphon which is connected to an axial tube on which the brush is mounted. This mounting tube must extend substantially throughout the length of the steam chamber in the drum and requires bearings at each end. Furthermore, the scheme presumes the elimination of spoiler bars on the dryer drum chamber wall, and therefore does not solve the problem for existing dryer drums equipped with spoiler bars and stationary siphons. Nor does the patented spoiler brush arrangement solve the problem where for any reason the spoiler bar and stationary siphon arrangement may be preferred for original equipment dryer drums in a new dryer assembly.